There are many prior art devices which measure part dimensions as a function of the gap or distance between spaced elements which contact the part. The sensing elements of these devices include various electrical, magnetic and thermally actuated transducers.
Simple coil and core transducers are sometimes used in gauging fixtures; however, the output signal of these gauges is of an exponential form and must be modified electronically to be useable.
Linear variable displacement transducers (LVDT) are magnetically operative devices which generate a linear output signal relative to displacement. U.S. Pat. No. 2,196,806 to Hoadley discloses an early embodiment of an LVDT. However, due to their mode of operation, these devices are typically of such configuration that they do not readily lend themselves to gauging applications in which measurements must be accurately taken from workpieces having particular configurations, e.g., a cylindrical bore, which are not easily accessible by the LVDT's.
Thermally actuated transducers are able to provide a linear output but the range of measurement is limited.
Several different structural combinations exist for communicating the displacement of spaced elements when contacting a part to a transducing means. Some of these combinations include interlocking gears, scissor type pivot pins and various complicated combinations. However, they are generally subject to wear and environmental influences that affect accuracy and reliability. Finally, the prior art devices are typically designed for gauging either internal or external dimensions exclusively.